Increased demand for digitally encoded speech and data services in cellular radio communications has resulted in a more efficient use of the spectrum through the development of a number of new digital modulation techniques for use over mobile radio channels. Several of these spectrally efficient modulation techniques use coherent quadrature modulation that can achieve greater than one bit per hertz efficiency.
This is the case in Group Speciale Mobile (GSM), the TDMA digital cellular radiotelephone standard being implemented in Europe. GSM uses gaussian minimal shift keying (GMSK) type modulation for the transmission of radiotelephone signals. The format of the data that is transmitted is illustrated in FIG. 3. The digitally sampled speech information (301 and 302) is located on both sides of a 26 bit training sequence or midamble (303). The training sequence is used by the radiotelephone receiving the signal to synchronize the radiotelephone in time with the signal. There are typically eight possible training sequences that can be used, each mobile radiotelephone having all eight received patterns stored in memory. The radiotelephone knows which training sequence to use by the base station color code (BCC) provided by the base.
Since GSM is a TDMA architecture communication system, it is time-slot oriented with eight time-slots per TDMA frame. The known active receive time-slot in a radiotelephone controls an R.sub.xAcq signal that is asserted high during the time-slot data is to be received. The R.sub.xAcq signal has a 1/8 duty cycle.
A typical transmitter apparatus used in a GMSK-type cellular radiotelephone system is illustrated in FIG. 4. The digitized speech symbols are first differentially encoded (401) yielding logic symbols (.delta..sub.i) before it is algebraically mapped (402) generating channel symbols (.alpha..sub.i). The algebraic mapping is as follows: EQU 0.fwdarw.+1.fwdarw.+.DELTA.f EQU 1.fwdarw.-1.fwdarw.-.DELTA.f
where .+-..DELTA.f is the change in frequency of the received signal.
These channel symbols are then GMSK modulated (403), which is best modeled as a gaussian low-pass filter followed by a voltage controlled oscillator, and mixed to the carrier freqency (404) before transmission. The transmit physical layer is further described in GSM Recommendation 05.04 Version 3.1.1, published January 1990.
The GMSK coherent quadrature phase modulation technique used presents significant problems due to the prohibitive costs and implementation complexity of a coherent quadrature phase receiver. There is a resulting need for a relatively low cost GMSK demodulator that allows the real-time reception and demodulation of a GMSK coherent quadrature phase modulated signal.